1. Field of the Invention
This invention resides in the field of air-breathing engines, and particularly combination engines that incorporate both a ramjet component and a low-speed booster component such as a rocket or a turbojet.
2. Description of the Prior Art
Air-breathing engines for hypersonic applications are known as “combined cycle” systems because they use a graduating series of propulsion systems in flight to reach an optimum travel speed or to leave the atmosphere altogether. Air-breathing engines use atmospheric air as a source of oxygen for combustion, as opposed to rockets which carry their own oxidizer. By using air captured from the atmosphere, air-breathing systems are several times more efficient than conventional rockets.
The thrust upon takeoff of a combined cycle engine and operation of the engine at low-to-moderate Mach numbers is achieved by a booster unit which consists of either rockets or turbojets or a combination of the two. Once the vehicle has reached a speed of Mach 2 or greater, the booster unit is replaced by a ramjet (which term is used generically herein to include “scramjet”) and acceleration is continued. The booster-to-ramjet transition is a critical stage in the operation of the engine since any loss of air flow through either engine during the transition can result in a loss of compression efficiency. The need to shift inlet air from the booster propulsion system to the high-speed propulsion system has resulted in large geometries that create flow resistance, surfaces and leading edges that produce complex shock waves, areas of separated or recirculating flow, and exposed moving parts that are vulnerable to damage.